1. Field of the Invention
The present invention pertains to holographic recording materials. More particularly the present invention pertains to composite graft polymers and their use in producing holographic optical elements with unique characteristics not achievable with currently available holographic recording materials.
2. Description of the Prior Art
Although a variety of materials and material systems have been utilized as holographic recording materials including silver halide, dichromated gelatin, ferroelectric crystals, thermoplastics, photo-resists and photopolymers, none can simultaneously fulfill the rigid requirements of high resolution, high sensitivity, low noise, ease of processing, and high transmissivity of short wavelengths. The following articles, incorporated by reference herein, describe some of these materials: J. W. Gladden, "Review of Photosensitivity Materials for Holographic Recording," Technical Report ETL-0128, U.S. Army Engineer Topographic Laboratory, April 1978; P. Hariharan, "Holographic Recording materials: Recent Developments," Optical Engineering Vol. 19, page 636, (1980); L. Solymar and D. J. Cooke, "Holographic Recording Materials", page 254-304 in Volume Holography and Volume Grating, Academic Press, N.Y., 1981.
For example, silver halide photographic emulsion for holography has shortcomings associated with scattering due to the granular structure of the recording medium, wet and multi-step chemical processing, maintaining the modulation transfer function at higher spatial frequencies, wavelength tunability, bandwidth, and instability at high temperatures. Photoelectric crystals, such as lithium niobate (LiNbO.sub.3), bismuth silicon oxide (BSO), bismuth germanium oxide (BGO), strasium barium nitrate (SBN) and PLZT ceramics have drawbacks related to the build-up of scattering which occurs during storage and readout owing to the formation of parasitic gratings. Reflectance phenomena are discussed in W. D. Corish, L. Young, J. Appl. Phy. 46, 1252 (1974) incorporated by reference herein.
Thermoplastics as holographic recording materials provide low efficiency holograms due to low resolution, low sensitivity, and low signal to noise ratio. This is discussed in T. Saito, T. Im Amura, J. Honda & J. Tsujnichi, J. Opt. 9, 325 (1978) incorporated by reference herein. Use of photoresists is limited to surface relief holography resulting in low efficiency surface holograms. Photopolymers appear to be useful as holographic recording materials. Potential problems of decreased signal to noise ratio, scattering, and low diffraction efficiency limit their performance as an effective recording polymer and lessen their desirability. Furthermore, with synthetic polymers, only limited bandwidth (40-50 nm), restricted wavelength tunability, and lower diffraction efficiency are possible. Recently Du Pont has presented (SPIE, Los Angeles, 1990) the results of their holographic photopolymer as a volume holographic recording material. Du Pont photopolymer has certain advantages such as dry processing and ease of fabrication and handling. This photopolymer, however, cannot be used where broad bandwidth is necessary in certain applications such as laser hardening. Moveover, it has a limited range of sensitivity. Similar problems exist in Polaroid's DMP-128 photopolymer which further requires wet processing and extra surface treatment of the finished hologram.
Gelatins, particularly dichromated gelatins (DCG), are often used as holographic recording materials. `Gelatin` is a product obtained by the partial hydrolysis of collagen derived from the skin, white connective tissues, and bones of animals. In collagen, three chains are twisted around one another to form a three-stranded helix, which is converted into water soluble protein from which water soluble gelatin is obtained. Chemically, gelatin is an albumin containing chains of amino acids. A single amino acid molecule roughly contains 2000 amino acid units, and there can be 20 or more different units in each molecule. Thus, the molecular weight of gelatin can range from 100,000-300,000, depending upon the amino acid moieties. The common molecular formula for an amino acid is: ##STR1## Every third atom of the amino acid chain has a side chain substituent while consecutive oxygen and amino groups for the long chain of the chemical structure of gelatin depend upon the particular amino acid unit involved: ##STR2## where R may be - H for glycine, R' may be ##STR3## for Valine and R" may be ##STR4## for Tyrosine. Basically gelatin is composed of a gly-prohydroxyproline repeat unit--which is more than 30% to 70% of the molecule depending upon the type of gelatin and its origin. Commonly found amino acid structures in gelatin are listed below including their individual proportions in the gelatin chain.
Table 1 illustrates the chemical structures of amino acids and Table 2 lists the amount of each amino acid found in gelatin. The percentage of amino acids depends upon the source of gelatin.
TABLE 1 __________________________________________________________________________ Chemical Structure Of Amino Acids Generally Found In Gelatin Polymer __________________________________________________________________________ ##STR5## ##STR6## ##STR7## Glycine (Gly) Alanine (Ala) Serine (Ser) M.W.: 75.07 M.W.: 89.09 M.W.: 105.09 ##STR8## ##STR9## ##STR10## Cysteine (CySH) Cystine (CysSCy) Aspartic Acid (Asp) M.W.: 121.16 M.W.: 240.30 M.W.: 133.10 ##STR11## ##STR12## ##STR13## Valine (Val) Threonine (Thr) Phenylalanine (Phe) M.W.: 117.15 M.W.: 119.12 M.W.: 165.19 ##STR14## ##STR15## ##STR16## Tyrosine (Tyr) Histidine (His) Methionine (Met) M.W.: 181.19 M.W.: 155.16 M.W.: 149.21 ##STR17## ##STR18## ##STR19## Glutamic Acid (Glu) Leucine (Leu) Isoleucine (Ileu) M.W.: 147.13 M.W.: 131.17 M.W.: 131.17 ##STR20## ##STR21## Lysine (Lys) Arginine (Arg) M.W.: 146.19 M.W.: 174.20 ##STR22## ##STR23## Hydroxyproline (Hypro) Proline (Pro) M.W.: 131.13 M.W.: 115.13 __________________________________________________________________________
TABLE 2 __________________________________________________________________________ Gelatin Molecular Formula RATIO OF AMINO ACID UNITS IN AMINO ACID % COMPOSITION MOL. WT. MOL. FORMULA __________________________________________________________________________ Cystine(s) 0.1 240 = 0.000417 0.000417 = 1 Cysteine(s) 0.1 121 = 0.000825 0.000417 = 2 Serine 0.4 105 = 0.0381 0.000417 = 9 Tyrosine* 0.5 181 = 0.00277 0.000417 = 7 Histidine 0.8 155 = 0.00517 0.000417 = 12 Methionine(s) 1.0 149 = 0.00670 0.000417 = 16 Isoleucine 1.4 131 = 0.0107 0.000417 = 26 Threonine 1.9 119 = 0.0160 0.000417 = 38 Phenylalanine 2.2 165 = 0.0134 0.000417 = 32 Valine 2.5 117 = 0.0214 0.000417 = 51 Leucine 3.2 131 = 0.0245 0.000417 = 59 Lysine* 4.1 146 = 0.0281 0.000417 = 67 Aspartic Acid* 6.6 133 = 0.0496 0.000417 = 119 Arginine* 8.1 174 = 0.0466 0.000417 = 112 Alanine 8.7 89 = 0.0975 0.000417 = 234 Glutamic Acid* 11.4 147 = 0.0775 0.000417 = 186 Hydroxyproline 14.1 131 = 0.108 0.000417 = 259 Proline 18.0 115 = 0.156 0.000417 = 374 Glycine 25.5 75 = 0.340 0.000417 = 815 TOTALS 110.6% 2431 __________________________________________________________________________ (s)Denotes sulfur containing amino acids. *Denotes the polar amino acids.
With regard to grafting generally, grafting of polymers has in the past been used to alter the mechanical properties such as tensile strength, impact strength, extensibility, etc. of a number of polymers. For example moisture permeability of nylon can be significantly reduced by grafting styrene on nylon whereas grafting of acrylonitrile on polyethelene film reduces gas permeability. Grafting of acrylonitrile on silicon rubber has improved the solvent resistance of silicon rubber. Other functionalities such as dyeability and adhesion can be enhanced through grafting. Compared to polystyrene or poly-1-butene, the polystyrene-graft-1-butene polymer exhibits much higher values of dielectric constant and dielectric loss factor, Petoraro, M., Beati, E., Severini, F., Chem. Ind (Milan) 42, 843 (1960). Grafting and crosslinking techniques also have been used to increase the hardness, durability, and wear resistance of rubber.
With regard to grafting of gelatin, a gelatin graft is a chemical modification of its molecular structure through extended derivatization of chemically active groups in gelatin molecules. These active groups are either the end groups or side chain groups. Deamination of amino groups of gelatin by nitrous acid and removal of guanidine (from arginine) by hypobromite oxidation has been carried out as discussed in A. W. Kenchington, Bichem J. 68, 458 (1958); and P. Davis in G. Staisby, ed., "Recent Advances in Gelatin and Glue Research", Pergamon Press, London, 1958, p.225 incorporated by reference herein. Phthalated and carbamoylated gelatin for the photographic industry and arylsulfonylated gelatin for microencapsulation are a few of the chemically modified gelatin formulations used for commercial purposes. These modified gelatins are discussed in U.S. Pat. No. 3,184,312 (assigned to Eastman Kodak); British Patent No. 1,075,952 (assigned to Gelatin and Glue Research Assoc.) incorporated by reference herein.
Gelatin also has been derivatized by epoxides, cyclic sulfones, and cyanamide. These derivatizations are discussed in Belgium Patent No. 672,906 (assigned to GAF Corporation); British Patent No. 1,033,189 (assigned to Kodak, Ltd.); and British Patent No. 1,100,842 (assigned to American Cyanamide) incorporated by reference herein. Grafting of acrylic polymers--through active double bonds--is disclosed in U.S. Pat. No. 3,291,611 (assigned to Swift & Co) incorporated by reference herein.
Dichromated Gelatin (DCG) exhibits ideal properties useful for volume phase holograms but there are significant potential drawbacks of DCG, and holographic optical elements based on it are easily affected by environmental changes such as temperature, pressure, wind, etc. Furthermore, high scatter, precipitation, and haze generally plague DCG coatings and holograms especially in shorter wavelengths. The need for a holographic material that combines the advantages of a desirable holographic material, yet avoids the drawbacks of many of the prior art materials is apparent.